Adjust the sampling rate in reports
Change the sample size to adjust reporting precision and speed.
When you view a sampled report, you have the option to adjust the sampling rate in order to increase accuracy and affect how quickly the report loads.
At the top of the report, below the date range selector, select Faster response, less precision or Slower response, greater precision. Your data automatically refreshes using the new setting. Your preference is saved across all of your reports, but resets after you close Analytics.